1. Field of the Invention
The present invention relates to a display device, in particular, to an art which can be applied effectively to a TFT liquid crystal display device.
2. Description of the Related Art
Conventionally, a liquid crystal display device provided with a liquid crystal display panel which encloses a liquid crystal between two substrates is known as one of display devices. The liquid crystal display device includes a so-called active-matrix liquid crystal display device.
The active-matrix liquid crystal display device possesses plural scanning signal lines, plural picture signal lines, plural active elements, plural pixel electrodes, and a counter electrode in the liquid crystal display panel. The active elements and the pixel electrodes are arranged in a matrix. The general active-matrix liquid crystal display device employs a TFT of MIS structure (including MOS structure) as the active element. The pixel electrode is coupled to one of a source and a drain of the TFT. A gate of the TFT is coupled to the scanning signal line. One of the source and drain of the TFT, which is not coupled to the pixel electrode, is coupled to the picture signal line.
In the liquid crystal display panel, the pixel electrode and the counter electrode are electrodes to drive the liquid crystal, and the arrangement method of the pixel electrode and the counter electrode is classified roughly into two methods. The first arrangement method arranges the pixel electrode in one substrate (hereinafter called a TFT substrate) of the two substrates, and the counter electrode in the other substrate (hereinafter called a counter substrate). The second arrangement method arranges the pixel electrode and the counter electrode in the TFT substrate.
The second arrangement method is further classified into two methods: an opposing arrangement to arrange the pixel electrode and the counter electrode in a plane on the same side of an insulating layer which is provided in the TFT substrate, and a laminating arrangement to laminate the pixel electrode and the counter electrode with an intervening insulating layer provided therebetween in the TFT substrate.
In the opposing arrangement to arrange the pixel electrode and the counter electrode in a plane on the same side of an insulating layer which is provided in the TFT substrate, for example, the pixel electrode and the counter electrode, each in a plane shape of comb, are arranged in an opposing manner in a plane, so that the teeth of the two comb-shaped electrodes may be placed alternately.
The liquid crystal display panel, in which the pixel electrode and the counter electrode in a shape of comb are arranged in an opposing manner, drives a liquid crystal generally by the method called a lateral electric field drive method (or simply called in-plane switching method). Therefore, the liquid crystal display panel is easy to realize a wide viewing angle and used for a liquid crystal television etc.
By the way, the active-matrix liquid crystal display device performs display of each pixel by writing to the pixel electrode a brightness level voltage signal currently applied to the picture signal line, in the period when the gate of TFT is ON. Therefore, it is desired to manufacture the TFT substrate so that the write current value of each TFT may become uniform.
However, the conventional manufacturing method of the TFT substrate is susceptible to generate a difference in the write current value of each TFT due to various factors. Therefore, there was a problem that brightness unevenness tends to occur due to insufficient writing of TFT.
As a method of reducing the insufficient writing of each TFT, a method of assuring sufficient writing by a three-level scanning signal is recently proposed, for example (refer to JP-A 2004-004876).
However, when the scanning signal is expressed in terms of a three-level signal as proposed by JP-A 2004-004876, a circuit for converting the scanning signal to a three-level signal is additionally needed in a drive circuit (a driver IC) coupled to a scanning signal line. Therefore, there arises a problem, for example, that the volume of the drive circuit becomes unfavorably larger, making it difficult to realize a liquid crystal display device with a narrower picture frame and what is worse, leading to a rise in the manufacturing cost.